CoQA: A Conversational Question Answering Challenge Siva Reddy author Danqi Chen author Christopher D Manning author 2019 text journal article Transactions of the Association for Computational Linguistics continuing MIT Press Cambridge, MA periodical academic journal reddy-etal-2019-coqa 10.1162/tacl_a_00266 https://aclanthology.org/Q19-1016/ 2019 7 249 266